Ink printing apparatuses may be used for single-color or multicolor printing to a printing substrate, including single sheet or belt-shaped printing substrate that can be made from various materials such as, for example, paper or a paper web. An example design of such ink printing apparatuses is illustrated in, for example, EP 0 788 882 B1. Ink printing apparatuses that, for example, operate according to the Drop-on-Demand (DoD) principle have, as a printing unit, a print head or multiple print heads with nozzle units comprising ink channels and activators. The activators—controlled by a printer controller—may excite ink drops in the direction of a printing substrate web. The ink drops are directed onto the printing substrate web in order to apply print dots there for a print image. The activators may generate ink drops thermally (bubble jet) or piezoelectrically.
The design of a print head that has (for example) a nozzle unit with piezoelectric activators is illustrated in U.S. Pat. No. 7,281,778 B2. The nozzle unit can include ink channels that end in nozzles arranged in a nozzle plate, and provides activators that are respectively arranged at an ink channel. The printing substrate web is directed past the nozzle plate. If printing should occur, the activators provided for the printing are activated by a printer controller, which activators thereupon subject the ink in the ink channels to pressure waves via which the ejection of ink drops from the nozzles in the direction of the printing substrate web is induced.
Given low print utilizations of the ink printing apparatus, not all nozzles of the ink print heads are activated in the printing process and many nozzles have downtimes (printing pauses) that can result in the ink in the ink channel of these nozzles not being moved. Due to the effect of evaporation from the nozzle opening, the danger exists from this that the viscosity of the ink then changes. This has the result that the ink in the ink channel can no longer move optimally and, for example, can no longer exit from the nozzle. In extreme cases, the ink in the ink channel dries completely and clogs the ink channel, such that a printing with this nozzle is no longer possible.
The drying of the ink in the nozzles may be prevented in that printing occurs from all nozzles within a predetermined cycle. This cycle may be set corresponding to the print utilization. Individual points may thereby be applied in unprinted regions of the printing substrate web, or print dot lines may be printed between print pages. These methods may lead to disruptions in the print image, in addition to unnecessary ink consumption and additional wear of the print heads.
A drying of the ink in the nozzles of a print head in its printing pauses represents a problem that may also be prevented in that a purge medium (e.g., ink or cleaning fluid) is flushed through all nozzles in a flushing process (also called purging) within a predetermined cycle. This purge cycle may be set corresponding to the print utilization as illustrated in, for example, EP 2 418 087 A1.
In order to improve the fixing of colored ink on the printing substrate web, a fixative can be applied to the regions of the printing substrate web that should be printed to with colored ink. The print regions on the printing substrate web may be determined using the print data. Before the printing, the coating of the printing substrate web may be implemented with a coating unit that may be designed corresponding to an ink print head. For example, if the coating unit has a nozzle plate with nozzles, only the respective print region may be specifically coated with fixative, wherein the coating quantity is also adjustable. Ink printing apparatuses that have such coating units are illustrated in U.S. Pat. No. 7,645,019 B2, U.S. Pat. No. 7,530,684 B2 or DE 100 59 573 A1, for example.
In inkjet printing, special transparent inks may be applied onto the printing substrate web (e.g., by a print head or a print bar made up of print heads) as fixative for the use cases indicated above. The basic function of this operation is to increase the surface tension of the printing substrate web so that the colored ink spreads better and the surfaces that are printed to are closed and produce a homogeneous effect. This is achieved by transferring salts, for example, onto the surface of the printing substrate web. The effect of this surface treatment of the printing substrate web is additionally that a larger quantity of color pigments from the colored ink remain on the surface and modify the color impression relative to an untreated surface.
However, the fixative alone cannot be detected on the surface of the printing substrate web because the fixative penetrates into the surface of the printing substrate web or on its own does not sufficiently modify the appearance of the printing substrate web in order to make it possible to differentiate printed locations from unprinted locations with certainty. This leads to the situation that—during the printing, for example—a check may not be made as to whether all nozzles of the print heads for the fixative are functioning. That is, due to optical properties of the fixative, nozzle errors of these print heads may not be directly detected. Nozzle errors (failures of nozzles or angular deviations in the flight of the fixative towards the printing substrate web) therefore may not be directly evaluated in the printing operation, for example, to introduce countermeasures such as purging.
The exemplary embodiments of the present disclosure will be described with reference to the accompanying drawings.